slirp: Allow non-local DNS address when restrict is off

This can be used to set a DNS server to be used by the guest which is
different from the one configured on the host.

Buglink: https://bugs.launchpad.net/qemu/+bug/1010484
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

net/slirp.c

@@ -456,7 +456,7 @@ static int net_slirp_init(NetClientState *peer, const char *model,
         error_setg(errp, "Failed to parse DNS");
         return -1;
     }
-    if ((dns.s_addr & mask.s_addr) != net.s_addr) {
+    if (restricted && (dns.s_addr & mask.s_addr) != net.s_addr) {
         error_setg(errp, "DNS doesn't belong to network");
         return -1;
     }
@@ -522,7 +522,7 @@ static int net_slirp_init(NetClientState *peer, const char *model,
             error_setg(errp, "Failed to parse IPv6 DNS");
             return -1;
         }
-        if (!in6_equal_net(&ip6_prefix, &ip6_dns, vprefix6_len)) {
+        if (restricted && !in6_equal_net(&ip6_prefix, &ip6_dns, vprefix6_len)) {
             error_setg(errp, "IPv6 DNS doesn't belong to network");
             return -1;
         }